1. Field of the Invention
The present invention relates to an optical fiber having a non-strippable thin coating disposed on a cladding.
2. Description of the Related Art
As shown in FIG. 5, an existing optical fiber ribbon is composed of a plurality of optical fibers 4 (normally, four to twelve fibers) and an outer coating 5. The optical fibers 4 are arranged in a row. The outer coating 5 is made of synthetic resin with a Young's modulus of 50 kgf/mm.sup.2 to 100 kgf/mm.sup.2 at room temperature. The outer coating 5 is disposed around the optical fibers 4. Each of the optical fibers 4 is composed of a core and cladding 1, a primary coating 2, and a secondary coating 3. The primary coating 2 is disposed around the cladding and made of synthetic resin with a Young's modulus of 0.1 kgf/mm.sup.2 to 0.5 kgf/mm.sup.2 at room temperature. The secondary coating 3 is disposed on the primary coating 2 and made of synthetic resin with a Young's modulus of 40 kgf/mm.sup.2 to 100 kgf/mm.sup.2 at room temperature. The coatings 2, 3 and 5 suppress a transmission characteristic from fluctuating against a lateral pressure and a temperature variation.
Each coating of the optical fiber ribbon has an optimum thickness so as to stably protect the characteristics of the core and cladding 1. For example, in the case of an optical fiber ribbon for use with a four single-mode (SM) fibers, the primary coating 2 and the secondary coating 3 are disposed on an SM optical fiber with a diameter of 125 .mu.m in such a manner that the diameter of the primary coating 2 becomes 180 to 200 .mu.m and the diameter of the secondary coating 3 becomes 230 to 250 .mu.m. In addition, the outer coating 5 is disposed in such a manner that the ribbon width and ribbon thickness become 1.10 mm and 0.38 mm, respectively.
As optical communications have advanced in recent years, an optical fiber cable that densely contains optical fibers has been required. Thus, the improvement of the fiber density of an optical fiber ribbon, which is a basic dimension of an optical fiber cable, has been demanded.
Moreover, in recent years, a planar lightwave circuit (PLC device) has been densely structured. From this point of view as well, an optical fiber ribbon with a high fiber density has been demanded. As shown in FIG. 6, when a high density PLC fiber array with a distance of 125 .mu.m between the center of the optical fibers is formed with two SM optical fiber ribbons with eight fibers 6 having a diameter of 250 .mu.m, the core and cladding 1 with an diameter of 125 .mu.m should be cross-bent. Thus, in addition to low workability, the bare fibers may contact in the fabrication process. Thereafter, the core and cladding 1 may break because of an aged deterioration due to a bend stress. Consequently, the reliability of the PLC device may deteriorate. Thus, a high density optical fiber ribbon for use with to a high density PLC device has been desired.
However, in the optical fiber ribbon, as described above, it is difficult to decrease the thickness of each coating for improvement of the fiber density since each coating is disposed with an optimum thickness necessary and sufficiently for stable characteristics of optical fibers. Thus, the optical fiber ribbon cannot satisfy such requirements.